Method of sintering chromium-alumina metal ceramics



METHOD OF SINTERING CHROMIUM-ALUMIN METAL CERAMICS No Drawing.Application April 28, 1954 Serial No. 426,297

2 Claims. c1. 75-206) This invention relates to an improved method ofsintering metal ceramics, particularly metal ceramics of thechromium-alumina type.

Metal ceramic articles, such as those of the chromiumalumina type areformed to the desired shape in the green (that is, unbaked) state. Theshaped articles, suitably supported, are then heated to sinteringtemperature in an inert, reducing, or non-oxidizing atmosphere, or in avacuum. This method is generally satisfactory for sintering most metalceramic articles, but is impractical for elongated, slender articlesover about twelve inches long. Sucharticles exhibit a marked tendency tofail by cracking. This is due to a decrease in strength of the articleduring early stages of sintering. Low thermal shock resistance of greencastings is but one of the reasons for such failures. At high sinteringtemperatures, supporting saggers distort and thus present unevenlongitudinal support for the articles. The effect of increasedsupport-spans occasioned by this distortion of the supports becomesespecially pronounced when long, thin, green castings are sintered atlow strength levels, and the elongated castings are incapable ofsupporting their own weight over the increased support length. As aresult, cracks are formed in the sintered articles before they have hada chance to gain the strength that comes with heat treatment. Theprevalence of these cracks in most instances exceeds 50% of the warecharged into the furnace.

It is, therefore, a primary object of this invention to provide animproved method of sintering metal ceramic articles of a wide variety ofshapes, such as rods, tubes and crucibles made from metal ceramicmaterial, which method reduces substantially the formation of cracksduring sintering.

The present invention is based on the discovery that the above defectsmay be minimized by preliminarily heat treating metal ceramic articlesbefore they are sintered. By heating the metal ceramic warepreliminarily at a relatively low temperature in an oxidizing furnaceatmosphere, extensive deleterious oxidizing and corrosive effects,heretofore thought to be unavoidable, are not experienced. Yetsufiicient strength is imparted to the ceramic ware during this initialheat treatment to enable them to withstand more rigorous thermaltreatrnent when later sintered to maturity. As a consequence, hightemperature furnace operations may then be carried out undernon-contaminating conditions without contracting the defects previouslyexperienced in the conventional methods of sintering. The termnon-contaminating conditions, as used hereinabove, is meant to includeany medium such as an inert, non-oxidizing, or reducing atmosphere, or avacuum, which will rninimze the possibility of chemical reactionoccurring between the article being treated and the surrounding medium.

In carrying out the method of the invention, green castings are preparedfrom metal ceramic material consisting, for example, basically of achromium alumina composition, preferably a composition havingapproximately 77% chromium and 23% alumina. The cast Ware are UnitedStates Patent dried and packed in undistorted supporting saggers, andare ready for preliminary firing. In the present instance the ware arepreferably supported in saggers made of alumina, although other types ofsupports may be used.

According to this invention, control of the furnace atmosphere is notnecessary in the initial phase of heat treatment of cast metal ceramicarticles. In this particular instance, a low temperature heatingoperation may be carried out in a suitable oxidizing medium such as air.Owing to the previously described hazards of heat treatment, thetemperature may well be gradually increased to a point considerablybelow sintering temperatures,

for example, 500 C. to 1000 0., preferably 750 C.

The period over which this preliminary heating temperature is reachedvaries with the accuracy with which the heat can be controlled. It cantake anywhere from less than an hour to as much as several hours toreach the desired tempertaure. For example, the low temperature heatingoperation can be effectively accomplished in less than one-half hour inan infra-red oven. On the other hand, this heat treatment may take overthree hours in a gas-fired furnace.

Following the preliminary heat treatment, the chromiurn-alumina articleis then appropriately sintered at a temperature in the range 1500 C. to1900 C., for example, at about 1600 C. Since it is subject to oxidationat such temperature, heating should be carried out undernon-contaminating conditions, i. e., in vacuo or in an inert or hydrogengas atmosphere.

Chromium-alumina ceramics heated preliminarily in an oxidizing medium inaccordance with this invention are provided with superior strength inthe form of green castings, and are found to exhibit remarkablyexcellent thermal shock resistance at sintering temperatures. This isborne out by the sharp decrease in the occurrences of cracks,particularly in the sintering of elongated metal ceramic articles. Forinstance, tests employing the method of the present invention show thatlong, thin thermowells made from chromium-alumina material andheat-treated in accordance with the novel practice hereindescribed, hadan extremely low incidence of cracking during sintering. Only 3% to 5%of them cracked during sintering, as opposed to the prior art occurrenceof more than 50%. Moreover, there is a significant reduction in thenumber of rejects in the production of metal ceramic ware, even when theware are supported on uneven or misaligned saggers during the sinteringoperation.

At radio frequencies, ware produced by the instant method exhibitincreased susceptance over similar ware in the original, as-castcondition. Further, ware manufactured by the instant method experience anegligible amount of shrinkage after pre-sinter treatment, the shrinkagebeing of the order of less than 1%. In contrast, normal shrinkage afterfull sintering is approximately 15%.

From the above it will be seen that the instant method comprisinginitially heating the green castings in an oxidizing atmosphere at arelatively low temperature, enables the castings to become sufiicientlystrengthened for sintering at elevated temperatures without theformation of cracks.

It will be understood that variations and modifications may be effectedwithout departing from the novel concepts of the present invention. 1 1

What is claimed is:

1. In a process for producing an article of substantially elongatedshape from metal ceramic material consisting essentially of chromium andalumina, wherein chromium constitutes the major constituent of saidmetal ceramic, the process of minimizing cracking during heat hardeningcomprising forming the article to said elongated shape in the greenstate, heating the article to a temperature between 500 C. and 1000 C.in an oxidizing atmosphere to increase the green strength of saidarticle, and then sintering said article in a non-contaminatingatmosphere until sintering of said article isj'complete.

2. In the process of sintering a green metal ceramic article over 12inches in length and consisting essentially of chromium and alumina,said chromium constituting the major constituent of said metal ceramic,wherein'the green article is supported on spaced supports, thecombination therewith of the improvement for reducing the development ofcracks during sintering due to low thermal shock resistance of saidarticle in the green state and distortion and wide spacing of saidsupports during sintering, said improvement comprising heating saidarticle prior to sintering in an oxidizing atmosphere up to atemperature between 500 C. and 1000 C. to increase the green strength ofsaid article, and then sintering said article in a medium which ischemically inert to said article at a temperature between 1500 C. and1900 0., whereby cracking of said article is minimized irrespective ofthe distortion and misalignment of the supports for said article duringthe sintering operation.

References Cited in the file of this patent UNITED STATES PATENTS1,663,553 Iredell May 27, 1928 2,051,972 Tigerschiold et a] Aug. 25,1936 2,207,708 Balke July 16, 1940 2,599,185 Lepp et al. June 3, 1952 102,698,990 Conant 61: a1. Jan. .11, 1955 OTHER REFERENCES Blackburn etal.: J. Am. Ceram. Soc. 32, 81 (1949).

Blackburn et al.: J. Am. Ceram. Soc. 34, 327 (1951).

Lidman et al.: Product Engineering, vol. 22, 147150 (October 1951).

Carter: Metallurgia, pages 8-13, January 1954.

1. IN A PROCESS FOR PRODUCING AN ARTICLE OF SUBSTANTIALLY ELONGATEDSHAPE FROM METAL CERAMIC MATERIAL CONSISTING ESSENTIALLY OF CHROMIUM ANDALUMINA, WHEREIN CHROMIUM CONSTITUTES THE MAJOR CONSTITUENT OF SAIDMETAL CERAMIC, THE PROCESS OF MINIMIZING CRACKING DURING HEAT HARDENINGCOMPRISING FORMING THE ARTICLE TO SAID ELONGATED SHAPE IN THE GREENSTATE, HEATING THE ARTICLE TO A TEMPERATURE BETWEEN 500*C. AND 1000*C.IN AN OXIDIZING ATMOSPHERE TO INCREASE THE GREEN STRENGTH OF SAIDARTICLE, AND THEN SINTERING SAID ARTICLE IN A NON-CONTAMINATINGATMOSPHERE UNTIL SINTERING OF SAID ARTICLE IS COMPLETE.